In the investigation of a nonlinear forced RL-varactor circuit

it is discovered that besides the period-doubling route to chaos

the system has also the frequency locking ability to its subharmonics. A frequency entrainment diagram of the system is obtained experimentally in a large scale of parameters

with a structure not found in literatures. The simplest model of the varactor is used to simulate the system. The bifurcation diagram and the phase-space orbits obtained from the computer agree well with those taken from the experiments. The mechanism of the chaos appearance is also studied. Possible applications of some results are presented.